The invention relates to a process for the recovery of ruthenium present in an aqueous nitric solution and relates to a process for recovering the radioactive ruthenium present in a nitric solution obtained by dissolving irradiated nuclear fuel elements.
Conventional processes for the reprocessing of irradiated nuclear fuel elements generally involve a first stage of dissolving the nuclear fuel by means of nitric acid, which leads to the obtainment of a nitric solution containing not only uranium and plutonium but also certain fission products, particularly ruthenium contained in by no means negligible quantities in irradiated fuels. Thus, one ton of irradiated uranium in a light water nuclear reactor contains approximately 2300 g of ruthenium.
Thus, considerable interest in attached to the recovery of ruthenium not only in order to ensure a satisfactory purification of the uranium and plutonium, but also due to the catalytic and photochemical metallurgical properties of ruthenium, which make it an element sought in numerous applications. However, in irradiated fuel processing processes the recovery of ruthenium causes certain problems, because during the first uranium and plutonium extraction cycle in an organic solvent, such as tributyl phosphate, the ruthenium fraction which is also extracted in this solvent is only re-extractable with difficulty and the solvents used consequently have a high residual activity.
It is also desirable to quantitatively recover the ruthenium before subjecting the nitric dissolving solution to a first uranium and plutonium extraction cycle.
The presently known processes for recovering ruthenium from nitric solutions comprise transforming the ruthenium complexes present in these solutions into volatile ruthenium tetraoxide RuO.sub.4. This ruthenium volatilization is generally carried out by means of ozone or other oxidizing agents such as anions, e.g. permanganates, dichromates, periodates, chromates, metal cations such as silver, sodium (IV), cobalt or lead dioxide.
The process for the recovery of the ruthenium present in a nitric solution is also known which consists of adding nitride ions to the solution and passing ozones through this solution in order to form and volatilize the ruthenium tetroxide.
However, these processes have certain disadvantages.
Thus, it is necessary to heat the solution to a temperature of about 95.degree. C. Moreover, said processes require the adding of catalysts to the nitric solution and only permit a quantitative recovery of ruthenium by being carried out over a relatively long period lasting approximately 12 hours.
A process for the extraction of ruthenium from aqueous solutions by means of organic solvents in the presence of nitride ions is also known.
It has also been envisaged to extract ruthenium from hydrochloric solutions by means of thiophosphoric acids.
However, these processes do not permit a quantitative recovery of the ruthenium from nitric solutions.